Field of the Invention
The present invention relates to a system for regulating anode-cathode spacing in a mercury-cathode electrolytic cell.
Production of chlorine and soda is obtained through the electrolysis of a saturated sodium chloride solution.
There are various types of electrolytic cells, amongst which the mercury-amalgam cathode cell is of great importance.
The cell (FIG. 1) consists of an iron tank on the bottom of which the mercury that forms the cathode surface flows. The anode surface is made up of a plurality of activated titanium electrodes (called D.S.D.) supported by frames that are moved manually or by microprocessor-controlled drive systems.
The electrical current is carried to the anode and the cathode by means of copper bars connected to an electrical power supply.
The above mentioned electrolytic cell, called a primary cell, is supplied with saturated sodium chloride brine which, when the current is passed, is decomposed and develops gaseous Cl.sub.2 at the anode, forming an amalgam of sodium and mercury at the cathode.
The sodium amalgam leaves the primary cell and enters a secondary cell, called a decomposer, filled with graphite and supplied with water.
On contact with graphite the amalgam reacts with the water and forms hydrogen, soda and mercury. The latter is returned to the primary cell by a pump.
A characteristic of the process in the primary electrolytic cell is that the cathode made up of mercury flowing on the bottom of the cell can vary in thickness both because of problems with the recirculating pump and particularly because of the scale that is deposited on the bottom of the cell due to the impurities contained in the brine. The thickness of the mercury can therefore vary from 1 to 20 mm depending upon process conditions.
In order to reduce power consumption (which is proportional to the anode-cathode distance) to a minimum, the anode-cathode distance must be kept between 1 and 3 mm, and in these operating conditions it becomes likely that short circuits will occur between the anode and the cathode, destroying the electrodes and generating explosive situations inside the cell.
For these reasons, the electrodes are supported by mobile frames controlled by microprocessors in order to maintain the anode-cathode distance constant.
For high performance to be achieved it is therefore of fundamental importance for the anode supporting system (frames) and particularly the drive system thereof to be precise in order to maintain the anode-cathode distance as constant as possible as the thickness of the mercury varies.
Various systems for adjusting the anode-cathode distance have been proposed, none of them without drawbacks.
One method consists of using four mechanical jacks disposed at the comers of the corresponding frame, driven two by two by geared motors. This system is very good as far as operating precision is concerned, but it can be used only for cells with few frames, that is of a large size, and it is very expensive.
Another method is a leverage system: to operate it needs strong levers with ovalized holes, thus with mechanical clearance that must be taken up, especially when inverting the direction of adjustment. The main defect is due to the fact that as the levers change in length during operation they introduce vertical movements of the leverage that are not proportional to the vertical movement of the frames. The control system is therefore not proportional to the actual distance between the electrodes, therefore computerized control is not possible.
Another system has pulleys with sprocket wheel drive chains: it introduces considerable clearance as the chains wear. Furthermore, this system does not afford the possibility of adjusting the anode-cathode planes, and therefore lacks precision.
Yet another system is with a torsion bar: a shaft that rotates with an angular movement mounted beneath the electrode frame. Adjustment is uncertain and imprecise especially because of the speed of raising, which, for successful adjustment, is required to be about 0.3-0.6 mm/sec with a precision of 1/10mm. It has the same defect as the leverage.